A common signaling method employed in the telephone network is dual-tone multifrequency ("DTMF") signaling. In this signaling scheme, pairs of tones are used to signal the digits 0 through 9, pound ("#"), star ("*") and the digits "A", "B", "C", and "D". For each pair of tones, one of the tones is selected from a low group of four frequencies and the other tone is selected from a high group of four frequencies. The correct detection of a signal requires both a valid tone pair and a correct timing interval. Other signals, such as an "alerting signal," use additional tone pairs, and may be used for special features such as caller identity delivery on call waiting ("CIDCW").
DTMF signaling is used both to set up a call and to control certain features such as call forwarding and conference calling. In some applications, it is necessary to detect DTMF signaling in the presence of speech (e.g., CIDCW), therefore it is important that the speech waveform is not interpreted as a valid signaling tone. For example, some customer premises equipment ("CPE") include the capability to receive caller identification information while the CPE (i.e., telephone) is in use (i.e., "off-hook"). For off-hook transmission of caller identification information (i.e., CIDCW), a CPE must detect, in the presence of speech, a CPE alerting signal ("CAS") as specified in Customer Premises Equipment Compatibility Considerations for the Voiceband Data Transmisson Interface (Bellcore SR-TSV-002476; December, 1992). The CAS signal is generally defined as an 80-85 ms dual tone, with nominal frequencies of 2130 Hz and 2750 Hz. Upon receiving the CAS tone, the called CPE temporarily mutes the CPE's microphone and speaker while the calling party's identity information is received.
Because the CAS tone frequencies (ie., 2130 and 2750 Hz) lie within the speech bandwidth, a CPE must undergo several tests to determine whether it can accurately detect a CAS signal in the presence of speech In particular, the CAS detector must operate in the presence of speech without incorrectly identifying the speech signal as a valid CAS, referred to as "talk-off" performance, and without missing a valid CAS due to the presence of speech, referred to as "talkdown" performance. Because CPEs commonly use a "hybrid" to interface to the public switched telephone network ("PSTN"), the transmitted speech, or near-end audio, may be coupled by the hybrid into the received signal path. The coupling of near-end audio into the received signal which may include a CAS signal, can cause the CPE's CAS detector to either miss a valid CAS signal or to incorrectly identify the presence of a CAS signal.
Therefore, what is needed in the art is a way of reliably detecting a CAS signal received on a telephone line while the line is carrying near-end audio traffic.